1. Field of the Invention
This invention relates in general to radar and sonar systems, and in particular to a circuit for correcting errors associated with a quadrature phase comparator for a radar or sonar system.
2. Description of the Prior Art
In some radar and sonar applications, doppler processing is used to enhance sensitivity to signals of interest and to suppress signals which are not of interest and are called clutter. Doppler processing is more effective if the phase and the amplitude of the return signal are detected. To preserve both phase and amplitude, two videos, I and Q, are detected. The I video is proportional to the amplitude of the return signal multiplied by the cosine of the phase difference between the return signal and a reference signal. The Q video is proportional to the amplitude of the return signal multiplied by the sine of that phase difference.
In a processing circuit, the I and Q video are created by a quadrature phase comparator. The phase comparator normally has a 90 degree power divider which receives a reference frequency input and produces two reference outputs. One of the reference outputs is shifted 90 degrees out of phase with the other. The phase comparator also has either a 0/0 degree or a 0/180 degree power divider which receives the return signal input and produces two in phase return signal outputs (0/0), or one output 180 degrees out of phase with another output (0/180). The two reference outputs and the two return signal outputs are applied to two double-balanced mixers. Each mixer receives one of the reference outputs and one of the return signal outputs and produces the sum and the difference of each to a low pass filter. The result is an I video output whose frequency is the difference between the reference and return signal input frequencies, and a Q video output which is identical in frequency to the I video output but shifted 90 degrees in phase.
In an actual phase comparator as described, there is an error called nonquadrature. The error results from I and Q videos that are not exactly 90 degrees out of phase. This nonquadrature error is one of the most common and most difficult to cure errors that is present in receiver systems which are trying to recover both the amplitude and the phase difference from a stable reference of a return signal. In a system which makes frequency analysis of the return signal, nonquadrature error will cause a false target called an image to appear whose doppler frequency or velocity is the same as that of a real target but of opposite sign. One simple measure of a nonquadrature error, after amplitude errors have been balanced out, is the image rejection; that is, the ratio of the power of the image or false target to the power of the true target. Under certain circumstances these images can degrade the performance of the overall system.
In the prior art, this nonquadrature error can be reduced to approximately one degree or less only by carefully matching the internal transformers and hand tuning during manufacturing. This increases the cost of manufacturing the phase comparator considerably.